Plastic bags and cigarettes are similar in many ways; both are, after use, commonly found man-made pollutants, on land, in water and in the air.
Both can kill.
However, while it is mandatory to have graphic health warnings on cigarette packs, there is no such requirement for plastic bags.
To discourage the use of single-use plastic bags, the bags can carry graphic images of entangled sea creatures, choked and suffocated marine creatures, and plastic pellets/waste from dissected stomachs.
This would then stigmatise the use of plastic bags as is appropriate for what is a global killer.